1. Field of the Invention
The invention relates to an X-ray system characterized by having a reduced size, and in particular, to techniques for providing and using the X-ray system.
2. Description of the Related Art
There is an interest in compact, low power consumption X-ray devices for a variety of purposes, including portable x-ray analytical instruments. Providing small form X-ray devices has, however, been a challenge.
For example, the size of conventional high voltage power supplies necessary to power X-ray equipment has constrained designers. This has been exacerbated by associated electrical insulation requirements. X-Ray tubes typically used in portable instruments require up to 60,000 Volts accelerating voltage and <1 mA of beam current. The most beneficial arrangement for portable x-ray instrumentation is a grounded anode x-ray source such that a negative high voltage is applied to a cathode end of the x-ray tube, while the output anode end is held at ground potential and presented to the sample. Operation for these types of portable x-ray fluorescence (XRF) instruments requires independent control of the accelerating voltage and the beam current.
Miniature x-ray tubes with a grounded anode design require up to 1 watt of power at one to two volts be applied to the thermionic filament in order sufficiently to emit electrons. The difficulty is that this low voltage, relatively high power signal needs to be isolated from ground potential by the full high voltage being applied to the cathode end of the x-ray tube. Traditional isolation transformers are bulky, generate excessive EMI and are unreliable. As they are situated in the high voltage region of the x-ray source, they contribute significantly to the overall size of the power supply due to electrical insulation requirements. The usual method of isolation is through the use of a transformer. Traditional transformers achieve isolation through independent insulating materials used in their construction. These materials have not historically been matched to the requirements of a miniature high voltage system for use with small x-ray tubes. The non-optimized transformers lead to increased size and wasted space in the design.
Traditional packaging schemes for these miniature power supplies and x-ray tubes use a metal enclosure to contain the insulating material, minimize the emitted electrical noise and reduce the chance of corona which can lead to a degradation of the insulating material over time. Because of the proximity of the metal case to the high voltage components of the power supply, space between the components must be filled with a high dielectric strength substance. Traditional fillers have included transformer oil or polymeric potting materials. These materials usually have a breakdown strength of 400-800 volts/mil requiring a substantial thickness in order to insulate the high voltage (up to 60,000 volts). For example, a 500 volt/mil material would need a minimum of about 0.120″ and normally a 100% safety margin is used resulting in a 0.240″ requirement for electrical insulation.
What are needed are methods and apparatus for providing x-ray generators that are smaller than previously achieved. Preferably, the techniques result in a versatile, low cost x-ray generator that may be used in instrumentation such as a portable x-ray fluorescence instrument.